of
dark colour, varying from green to very deep brown or even black. The
third variety is one in which iron replaces aluminium; it is called
Chlorospinel from its green colour, this colour being due to the
presence of traces of copper. The fourth variety, Picotite, has
replacements by both iron and chromium ; its chemical formula may be
expressed (MgFe)O, (AlCr)203.
When
heated the red varieties become brown ; on cooling again the red colour
is restored, the mineral passing through green and colourless stages.
The
distribution of Spinel is very similar to that of Ruby and Sapphire.
Thus it is found in Burma, in the gem gravels of Ceylon, and in New
South Wales. Other localities are Balachan (or Badakshan) in Usbec
Tartary, Pegu, Mysore, and Minas Novas in Brazil. In Sweden it is found
at Aker in Sodermanland in blue crystals (Akerite).
Spinel
is cut either as a brilliant or in the step pattern; when cut it may be
mistaken for several other minerals; thus the Ruby Spinel and some of
the Garnets are very similar, both being singly refracting, but Garnet
has a higher specific gravity, and it is also less hard than Spinel.
From red Topaz the Balas Ruby may be distinguished by the Topaz being
doubly refracting and dichroic, and also more strongly charged with
electricity when rubbed. The word " Balas " is said to be derived from
the place name Balachan. Spinel and Corundum of a violet colour can be
distinguished by the single refraction of the former and the dichroism
of the latter.
Spinels
take a high polish when cut, and when large their value is
considerable. The Ruby Spinel when above three carats approaches the
Ruby in value, but below that
